drm/i915: Enable pipe gamma for the overlay

We pass the plane data through the pipe gamma for all the other
planes. Can't see why we should treat the overlay differently,
so let's enable pipe gamma for it as well.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191028113036.27553-5-ville.syrjala@linux.intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>